1 Experimental Determination of Solubility Constants of Saponite at 2 Elevated Temperatures in High Ionic Strength Solutions, 3 Revision 1
4
5 Yongliang Xiong1
6 7 Department of Nuclear Waste Disposal Research & Analysis, 8 Sandia National Laboratories (SNL), 1515 Eubank Boulevard SE, Albuquerque, NM 9 87123, USA 10
1 Corresponding author, e-mail: [email protected].
1 11
12 ABSTRACT
13 Saponite occurs in a wide range of environments from hydrothermal systems on
14 the Earth to surface deposits on Mars. Of practical importance is that Mg-saponite forms
15 when glasses for nuclear waste are altered in Mg-bearing aqueous solutions. In addition,
16 saponite is favorably considered as candidate buffer materials for the disposal of high-
17 level nuclear waste and spent nuclear fuel in harsh environments. However, the
18 thermodynamic properties, especially for Mg-saponites, are not well known. Here the
19 author synthesized Mg-saponite (with nitrate cancrinite) following a previously reported
20 procedure and performed solubility experiments at 80 oC to quantify the thermodynamic
21 stability of this tri-octahedral smectite in the presence of nitrate cancrinite. Then, in
22 combination with the equilibrium constant at 80 oC for the dissolution reaction of nitrate
23 cancrinite from the literature, the author determined the solubility constant of saponite at
24 80 oC based on the solution chemistry for the equilibrium between saponite and nitrate
25 cancrinite, approaching equilibrium from the direction of supersaturation, with an
26 equilibrium constant of –69.24 ± 2.08 (2s) for dissolution of saponite at 80 oC.
27 Furthermore, the author extrapolated the equilibrium constant at 80 oC to other
28 temperatures (i.e., 50 oC, 60 oC, 70 oC, 90 oC and 100 oC) using the one-term
29 isocoulombic method. These equilibrium constants are expected to find applications in
30 numerous fields. For instance, according to the extrapolated solubility constant of
31 saponite at 50 oC and 90 oC, the author calculated the saturation indexes with regard to
32 saponite for the solution chemistry from glass corrosion experiments at 50 oC and 90 oC
33 from the literature. The results are in close agreement with the experimental
2 34 observations. This example demonstrates that the equilibrium constants determined in
35 this study can be used for reliable modeling of the solution chemistry of glass corrosion
36 experiments.
37
3 38 INTRODUCTION
39
40 Saponite, a tri-octahedral smectite, occurs in a wide range of environments. For
41 instance, it occurs in igneous rocks as hydrothermal alteration products (e.g., Kohyama et
42 al., 1973; April and Keller, 1992; Garvie and Metcalf, 1997), and in metamorphosed
43 dolomitic limestone as hydrothermal products (e.g., Post, 1984). It also occurs as lake
44 sediments and in volcanic rocks on seafloor (e.g., Desprairies et al., 1989; Cuevas et al.,
45 2003; Kadir and Akbulut, 2003). Its occurrence in the Martian meteorites and its
46 proposed occurrence in Mawrth Vallis region and Yellowknife Bay, Gale Crater on Mars
47 (e.g., Bishop et al., 2013; Hicks et al., 2014; Bristow et al., 2015) generated enormous
48 scientific interest. Most recently, saponite was also observed in an analog study of the
49 hydrothermal alteration in the martian subsurface (e.g., Sueoka et al., 2019; Calvin et al.,
50 2020).
51 Additionally, of practical importance and therefore particular interest is that
52 saponite is relevant to the geological disposal of nuclear waste, as Mg-saponite has been
53 observed as an alteration product when a borosilicate glass for nuclear waste is corroded
54 (Thien et al., 2010) in Mg-containing solutions, as detailed below.
55 Mg-bearing groundwaters are common in the disposal concepts in various rock
56 formations. For instance, salt formations have been recommended for nuclear waste
57 isolation since the 1950’s by the U.S. National Academy of Science (1957). Because of
58 their excellent properties of high heat conduction, low permeability and a propensity to
59 self-seal, rock salt is a viable candidate for a nuclear waste repository. Salt formations
60 contain brines, which have relatively high concentrations of magnesium (Nishri et al.,
4 61 1988; Schussler et al., 2001; Xiong and Lord, 2008). For instance, the Q-brine at Asse,
62 Germany, has a magnesium concentration of 4.47 mol•kg–1 (Nishri et al., 1988; Schussler
63 et al., 2001). When high level waste (HLW) glass is corroded in Mg-rich brines, Mg-
64 bearing phyllosilicates form as part of the secondary phase assemblage (Strachan, 1983;
65 Strachan et al., 1984; Grambow and Muller, 1989; Maeda et al., 2011). Although
66 previously, and tentatively, identified as sepiolite (a member of the palygorskite family),
67 more recent investigations have identified trioctahedral smectites, such as saponite with
68 various stoichiometries, e.g., (½Ca,Na)0.66Mg6[(Si7.34Al0.66)O20](OH)4(e.g., Abdelouas et
69 al., 1997; Thien et al., 2010; Zhang et al., 2012; Fleury et al., 2013; Debure et al., 2016;
70 Aréna et al., 2017) as a reaction product. Note that in these more recent investigations,
71 the concentration of Mg in solution is relatively low (milli-molal range), in accord with
72 the dilute nature of pore water chemical compositions recovered from silicate-dominated
73 prospective repositories (e.g., Gaucher et al., 2009). Other investigations have shown
74 that when Mg-bearing glass, such as the British Magnox compositions, is altered in dilute
75 aqueous solutions the released Mg also leads to formation of Mg-smectite phases (e.g.,
76 Curti et al., 2006; Thien et al., 2012; Harrison, 2014). It is likely, therefore, that saponite
77 could be a thermodynamically stable phase that precipitates when Mg-bearing solutions,
78 both dilute and concentrated, contact silicate glass for a sufficient length of time.
79 When saponite forms, it may impact the near-field chemistry of a geological
80 repository in various rock formations including salt formations by: (1) Controlling the
81 chemical compositions, including hydrogen ion concentrations, of the contacting
82 solutions, and (2) Forming a secondary mineral assemblage that will either enhance for
83 certain periods (Lucksheiter and Nesovic, 1998; Curti et al., 2006; Fleury et al., 2013;
5 84 Aréna et al., 2016, 2017), or retard (Grambow and Strachan, 1984; Frugier et al., 2005) or
85 both at different periods (Thien et al., 2012; Harrison, 2014), the dissolution rate of
86 glasses, and impact the retention of hazardous (radioactive) constituents after release
87 from the glass (e.g., Abdelouas et al., 1997).
88 In addition, because saponite has the swelling and sorption properties similar to
89 those of montmorillonite, saponite has also been proposed as a candidate buffer material
90 for HLW and spent nuclear fuel disposal in harsh environments (e.g., Yang et al., 2014).
91 This is because tri-octahedral smectites such as saponite have higher thermal and
92 chemical stabilities than di-octahedral smectites such as montmorillonite, and are less
93 susceptible to alteration in harsh environments (e.g., Eberl et al., 1978; Güven, 1990).
94 Because the phyllosilicates that form on the glass surface are typically not well
95 crystallized, and phase characterization efforts by X-ray diffraction are often technically
96 challenging, some investigators have resorted to using solution modelling to infer the
97 identity of candidate phases (c.f., Grambow and Müller, 1989). For this strategy to be
98 viable, knowledge of the thermodynamic properties of eligible phases, such as saponite,
99 is crucial. However, the thermodynamic properties of saponite, especially Mg-enriched
100 saponite, are not well-defined, and few studies have attempted to obtain them via glass
101 corrosion experiments (Aréna et al., 2017; Debure et al., 2016). For instance, in a recent
102 study, Aréna et al. (2017) tried to calculate the solubility constant of saponite (they called
103 it Mg-smectite) via geochemical modeling of their glass corrosion experiments.
104 However, their experiments were not designed, nor were they ideal, for determining the
105 solubility constants of saponite. Aluminum concentrations were very low in their
106 experiments, hampering modelling efforts. Hence, several compensating hypotheses
6 107 were constructed to estimate aluminum concentrations, including: (1) that aluminium
108 concentrations were equal to the quantification limit of the instrument (ICP-AES); (2)
109 aluminum concentrations were equal to the quantification limit divided by 1000; and (3)
110 aluminium was depleted stoichiometrically from saponite. These assumptions may not
111 be warranted.
112 Without accurate knowledge of the thermodynamic properties of saponite, it is
113 difficult to make reliable and accurate predictions for the evolution of chemical
114 compositions of the solutions containing Mg interacting with glass. Accurate knowledge
115 of its thermodynamic properties is thus the prerequisite for modeling glass corrosion in
116 Mg-bearing solutions, both at high and low ionic strength.
117 The objective of this study is to determine experimentally the solubility constants
118 of hydrous saponite at elevated temperatures under well constrained conditions. The
119 author first synthesized saponite (from nitrate cancrinite) according to an established
120 methodology (see Experimental Methods). Because hydrogen ion concentrations are a
121 key parameter governing the dissolution of saponite, the author first measured pH and
122 then applied correction factors obtained at elevated temperatures (Kirkes and Xiong,
123 2018). With proper knowledge of the hydrogen ion concentrations and the
124 concentrations of Al, Mg, Na and Si in solution, the author calculated the equilibrium
125 quotients. Then, the appropriate activity coefficient model is applied to extrapolate
126 equilibrium quotients at certain ionic strengths to infinite dilution. This model allows us
127 to then assess the solubility of saponite in previously reported experiments. It is shown
128 that an accurate log K value can be used to understand glass corrosion in Mg-bearing
129 solutions under a variety of geochemical conditions.
7 130
131 EXPERIMENTAL METHODS
132 In this study, the chemicals used for synthesis of the starting material were ACS
133 reagent grade chemicals. The synthesis followed the procedure of Shao and Pinnavai
134 (2010) with some modifications. In the work of Shao and Pinnavai (2010), the sources
135 for Al, Mg, Na and Si, were Al(NO3)3•9H2O, Mg(NO3)2•6H2O, and water glass solution
136 (containing 27 wt.% Si and 14 wt.% NaOH), respectively. In this work, the source for Si
137 is Na2SiO3•9H2O, and NaOH was used to control the pHm conditions. Other reagents are
138 the same as those in Shao and Pinnavai (2010). Deionized water (DI) with >18.3 MΩ·cm
139 used in the experiments was purged with high purity Ar gas for a minimum of one hour
140 to remove dissolved CO2, following the procedure of Wood et al. (2002) and Xiong
141 (2008).
142 A supersaturation experiment was conducted at 80.0 ± 0.5 oC as follows. First,
143 the author synthesized a mixture of saponite and nitrate cancrinite at 90 oC following the
144 procedure of Shao and Pinnavai (2010). The author kept the synthesized solid mixture
145 together with the mother solution in the same container in an oven at 90 oC for 7 days.
146 Then, the author transferred the container to another oven at 80 oC. Because aluminum
147 silicates such as zeolites exhibit prograde solubility (i.e., higher solubility at higher
148 temperatures) (Xiong, 2013), it is considered that the experiment initially kept at 90 oC
149 for some time, and then remained at 80 oC, approaches the equilibrium from the direction
150 of supersaturation.
151 Solution samples were periodically withdrawn from the experiments. Before each
152 sampling, pH readings were taken for each experiment. In each sampling, about 3 mL of
8 153 solution was taken from each experiment, and the solution samples were filtered through
154 a 0.2 µm filter, and transferred into pre-weighed 10 mL Grade A volumetric flasks. After
155 filtration, masses of each solution sample were determined with a balance that is precise
156 to the fourth decimal place. Samples were then immediately acidified with 0.5 mL of the
157 Optima® Grade concentrated HNO3 from Fisher Scientific, and diluted to 10 mL with DI
158 water. Chemical analyses for sodium, magnesium, aluminum, and silica were performed
159 using a PerkinElmer Optima 8300 Dual View (DV) ICP-AES.
160 The pH readings were measured with an Orion-Ross combination pH glass
161 electrode, coupled with an Orion Research EA 940 pH meter that was calibrated with
162 three pH buffers (pH 4, pH 7, and pH 13) at the experimental temperature of 80 ± 0.5oC.
163 The pH scale used in this work is the concentration scale (Mesmer and Holmes, 1992),
164 denoted as pHm, which is a negative logarithm of hydrogen ion concentration on a molal
165 scale. The pH readings are converted to pHm, according to the following equation (Xiong
166 et al., 2010),
167
168 pHm = pHob + Am = pHob + AM – log Q
169
170 where Am and AM are correction factors on a molal scale and a molar scale, respectively,
171 and Q is a conversion factor from molality to molarity. In this study, the expression of
172 correction factors as a function of ionic strength at 80 oC determined for NaCl solutions
173 from Kirkes and Xiong (2018) is used to calculate the correction factors for the ionic
174 strengths of the experiment.
9 175 Solid phases were analyzed using a Bruker D8 Advance X-ray diffractometer
176 with a Sol-X detector. XRD patterns were collected using CuKa radiation at a scanning
177 rate of 0.667o/min for a 2q range of 10–90o.
178 Major elemental compositions (Na2O, Al2O3, MgO and SiO2) of the saponite
179 crystals were determined by electron microprobe analysis (EMPA) at Sandia National
180 Laboratory, Albuquerque. Analyses were obtained using a JEOL JXA-8530F hyperprobe
181 electron probe micro analyzer with a field emission electron gun. The beam conditions
182 were <1 µm spot size, 15 keV accelerating potential and a current of 20 nanoamps.
183 Concentrations (wt.% oxide) were determined using Wavelength Dispersive
184 Spectrometry (WDS) to count on separate X-ray peaks judiciously chosen for efficiency,
185 performance and lack of interference. Background offsets were chosen at least 500 steps
186 away from the peak centers. Concentrations were quantified by comparing counts from
187 simple silicate standards whose chemical compositions are known. A table that lists the
188 element, the lower limit of detection, or LLD (wt. %), the crystal diffractometer and the
189 counting times is provided (Appendix A).
190 Concentrations of the major elements in saponite were determined by averaging
191 27 points. Adsorbed water was determined by loss on ignition (LOI) by heating a sample
192 to 700 °C for one hour using a Netzsch STA 409 thermal gravimetric analyzer (TGA).
193 The chemical formula of the saponite was calculated on the basis of O20(OH)4 per
194 formula unit.
195 SEM and EDS analyses were performed using a JEOL JSM-5900LV scanning
196 electron microscope (SEM) imaging system coupled with a NORAN System 7 Spectral
197 Analysis System for energy dispersive spectroscopy (EDS).
10 198
199 RESULTS
200 XRD pattern of the synthesized saponite is presented in Figure 1. As is shown,
201 this pattern is in reasonable agreement with that for saponite 15Å (PDF 00-013-0086)
202 (Figure 1). Notice that saponite 15Å is a pure end-member of magnesium saponite with a
o 203 chemical formula of Mg3(Si, Al)4O10(OH)2•4H2O. The peak at 2q ~60 , characteristic of
204 saponite, is present in the saponite synthesized in this work (Figure 1), which was also
205 observed in saponite synthesized at 160 oC by other researchers (i.e., He et al., 2014).
206 The d-spacings of the major peaks for the synthesized saponite compare well with those
207 of saponite 15Å standard (see Appendix B). In the synthesized saponite, there are some
208 concentrations of sodium, as indicated by EMPA (Table 1) and SEM-EDS analyses
209 (Figure 2). As my synthesis method follows that of Shao and Pinnavaia (2010), in which
210 it was known that there was nitrate cancrinite in addition to saponite, the two extra peaks
211 at 2q = 13.97o and 27.48o in Figure 1 can be attributed to nitrate cancrinite (Hund, 1984;
212 Liu et al., 2005).
213 Interestingly, the flower-like morphology of the synthetic saponite produced in
214 this study (see Figure 2A, 2B, 2C), is very similar to that observed for saponite
215 precipitated on the surface of vitrified intermediate level nuclear waste corroded in a
216 calcium-rich, magnesium-bearing solution (see Figure 4b in Utton et al., 2013). The
217 saponite in Utton et al. (2013) has a general formula of
218 Ca0.25(Mg,Fe)3[(Si,Al)4O10](OH)2•n(H2O). The calcium in the saponite reported in Utton
219 et al. (2013) comes from the high Ca solutions that reacted with glass. In the corrosion
–1 o 220 experiments of the nuclear waste glass in 0.016 mol•kg MgCl2 at 50 C performed by
11 221 Thien et al. (2010), the morphology of the corrosion product saponite (see Figure 1 in
222 Thien et al., 2010) is also similar to that of the synthetic saponite in this study. In Thien
223 et al. (2010), the stoichiometry of the saponite is
2+ 224 Na0.39(Mg2.25Li0.06Al0.06Fe0.06M0.25)[(Si3.23Al0.77)O20](OH)4, where M is a cation as Ni or
225 Mn2+. This stoichiometry is similar to that of the saponite synthesized in this study (see
226 below).
227 According to EPMA analysis, the stoichiometry of saponite synthesized in this
228 work is Na0.95(Mg5.90Al0.06)[(Si7.07Al0.93)O20](OH)4. The structural formula is calculated
229 by normalizing cation compositions to a theoretical structure containing O20(OH)4. The
230 total weight loss (loss on ignition, LOI) up to 700 oC is 19 wt%, similar to that for the
231 natural saponite from California (Post, 1984), which has a total weight loss of 16.66 wt%
232 (Table 1). The stoichiometry of the natural saponite from Ballarat, California, is
III 233 calculated as (Ca,Na,K)0.75(Mg5.17Al0.31Fe 0.13)[(Si7.55Al0.45)O20](OH)4 (Table 1). The
234 natural saponite from Milford, Utah, has a LOI of 17.30 wt% (Cahoon, 1954), and its
III 235 stoichiometry is calculated as (Ca,Na,K)0.42(Mg5.72Al0.19Fe 0.02) [(Si7.50Al0.50)O20](OH)4
236 (Table 1). In addition, the stoichiometry of the natural saponite from Allt Ribhein, Skye,
III 237 is (Ca,Na, K)1.02(Mg5.84, Mn0.01, Al0.03, Fe 0.08)[(Si6.99Al1.01)O20](OH)4 (MacKenzie,
238 1957) (Table 1). Therefore, the stoichiometry of natural saponite varies to some degree.
239 The stoichiometric coefficients for Mg are similar in saponite from these localities,
240 suggesting they belong to Mg-saponite. Notice that the stoichiometry of the synthetic
241 saponite is similar to that from Allt Ribhein, Skye, in terms of Si, Al, and Mg.
242 In this study, the solubility experiment at 80 ± 0.5 oC was performed from the
243 direction of supersaturation to approach equilibrium (see Experimental Methods section
12 244 for details). The experimental results for pHm, sodium, total magnesium, total aluminum,
245 total silica, nitrate and hydroxyl concentrations are presented in Table 2 and are displayed
246 in Figure 3. It is clear from Figure 3 that steady state conditions were achieved for all
247 solutes in terms of concentration with deviations generally ≤ 25% on a linear scale. For
248 instance, the sodium concentrations remain between 1.38 and 1.44 mol•kg–1, a deviation
249 of 4%. Note that Al and Si usually vary reciprocally for the dissolution of Al-silicates,
250 and it is appropriate to assess their variations using the solubility product of both Al and
251 Si (i.e., mSAl × mSSi) (Xiong, 2016). As nitrate cancrinite and saponite are in equilibrium
252 in our experiment, the equilibrium constant for the assemblage of nitrate cancrinite and
253 saponite can be determined, based on the experimental results.
254 The equilibrium between saponite and nitrate cancrinite in alkaline solutions can
255 be written as follows:
256
– + – 257 Na0.95Mg5.90[(Si7.07Al0.99)O20](OH)4 + 2NO3 + 7.05Na + 5.01Al(OH)4
2+ – 2– 258 ⇌ Na8[Al6Si6O24](NO3)2•4H2O + 5.90Mg + 9.62OH + 1.07H2SiO4 + 2.14 H2O(l)
259 (1)
260 The equilibrium quotient for Reaction (1) is:
261
5.90 1.07 9.62 ()(mm22+´´-- )() m Mg H24 SiO OH 262 Q1 = 7.05 2 5.01 (2) ()(mmm+´´-- )( ) Na NO34Al() OH
263
264 The equilibrium constant at infinite dilution is:
265
13 5.90 1.07 9.62 2.14 ()(gg22+´´´-- )()() gaHO 0 Mg H24 SiO OH 2 266 KQ11=´ 7.05 2 5.01 (3) ()ggg+´´ (-- )( ) Na NO34Al() OH
267
268 In the above equations, mi denotes a concentration for the i-th species on a molal scale,
–1 269 mol•g ; gi an activity coefficient for the i-th species; and aH2O activity for water. The
270 equilibrium quotient for Reaction (1) at 80 oC is listed in Table 3.
271 Similarly, the dissolution reaction for nitrate cancrinite in alkaline solutions can
272 be expressed as:
273
– 274 Na8[Al6Si6O24](NO3)2•4H2O + 12OH + 8H2O(l)
+ – 2– – 275 ⇌ 8Na + 6Al(OH)4 + 6H2SiO4 + 2NO3 (4)
276 The equilibrium quotient for Reaction (4) can be expressed as:
277
82 6 6 ()()(mm+´´--- m )( ´ m2 ) 278 Q = Na NO342Al() OH H SiO4 (5) 4 ()m 12 OH -
279
280 The corresponding equilibrium constant at infinite dilution is:
281
82 6 6 ()()(gg+´´--- g )( ´ g2 ) 0 Na NO342 Al() OH H SiO4 282 KQ44=´ 12 8 (6) ()()g - ´ a OH HO2
283
0 284 Bickmore et al. (2001) determined a value of log10 K = –36.2 ± 0.6 (2s) for
o 0 285 Reaction (4) at 89 C. Lichtner and Felmy (2003) provided a value of log10 K = –39.03
14 286 at 75 oC. The linear interpolation of these two values leads to a value of –37.99 ± 0.70
287 (2s) for Reaction (4) at 80 oC (Table 3). This value is used to retrieve the solubility
288 constant for saponite at 80 oC as detailed below.
289 In our thermodynamic calculations for the activity coefficients for Equations (3)
290 an (6), we use the computer code EQ3/6 Version 8.0a (Wolery et al., 2010; Xiong, 2011).
291 In all calculations, the activity of all solid phases is assumed to be unity. The EQ3/6 code
292 has been successfully used as a modeling platform in a number of previous studies at
293 ambient temperature (e.g., Xu et al., 1999; Kong et al., 2013; Xiong, 2015, 2020) and at
294 elevated temperatures up to 523.15 K (e.g., Xiong, 2013, 2014, 2016).
295 The database used for thermodynamic calculations is DATA0.YPF (Wolery and
296 Jarek, 2003), which utilizes the Pitzer model for calculations of activity coefficients of
297 aqueous species. The database was modified with the addition of the Pitzer parameters
298 and equilibrium constants for Al and Si species from Xiong (2013 and 2014).
299 Based on the experimental data, we first calculated the equilibrium constant for
300 Reaction (1) (Table 3). The equilibrium constant for Reaction (1) at infinite dilution and
301 80 oC is –31.25 ± 1.96 (2s) in logarithmic units (Table 3).
302 The combination of Reactions (1) and (4) leads to,
303
– 304 Na0.95Mg5.90[(Si7.07Al0.99)O20](OH)4 + 2.38OH + 5.86H2O(l)
+ 2+ – 2– 305 ⇌ 0.95Na + 5.90Mg + 0.99Al(OH)4 + 7.07H2SiO4 (7)
306
307 The equilibrium quotient for Reaction (7) can be expressed as:
308
15 0.95 5.90 0.99 7.07 ()(mm++´´22 )( m-- )( ´ m ) 309 Q = Na Mg Al() OH 42H SiO4 (8) 7 ()m 2.38 OH -
310
311 The corresponding equilibrium constant at infinite dilution is:
312
0.95 5.90 0.99 7.07 ()gg++´´ (22 ) ( g-- ) ´ ( g ) 0 Na Mg Al() OH 42H SiO4 313 KQ77=´ 2.38 5.86 (9) ()()g - ´ a OH HO2
314
315 According to the equilibrium constants for Reactions (1) and (4), the equilibrium constant
o 316 for dissolution of saponite at 80 C in alkaline solutions (i.e., pHm ~12.58), represented
317 by Reaction (7), is computed to be –69.24 ± 2.08 (2s) in logarithmic units (Table 3). The
318 quoted uncertainty includes the error propagations.
319
320 DISCUSSIONS
321 Debure et al. (2016) conducted three glass corrosion experiments with the
o 322 International Simple Glass (GISG) at 90 C. The first one involved pure water, the second
–3 -3 323 experiment used 8×10 mol•dm MgCl2 as the Mg-source, and the third corrosion
324 experiment was conducted in the presence of hydromagnesite. In their experiments with
325 MgCl2 and hydromagnesite, it was observed that Mg-phyllosilicates (saponite) were
326 formed as white crusts on the glass powder surface, whereas no Mg-phyllosicates were
327 observed in the experiment with pure water. Because, for their corrosion experiment
328 with hydromagnesite, they determined a complete set of chemical compositions for the
329 solution, that experiment is an appropriate candidate to test the solubility constant of
16 330 saponite determined in this study. In the test, this author calculates the saturation index
331 for saponite at 90 oC with the stoichiometry determined in this study for the experimental
332 solution in Debure et al. (2016) at the same temperature.
333 In the calculations, the author first extrapolated the solubility constant of saponite
334 determined at 80 oC to the experimental temperature of 90 oC in Debure et al. (2016). In
335 the extrapolation, the author uses the one-term isocoulombic approach (Gu et al., 1994)
336 for the following semi-isocoulombic reaction, utilizing ionization of water as a model
337 substance to balance the charges for the reaction:
338
– + 339 Na0.95Mg5.90[(Si7.07Al0.99)O20](OH)4 + 15.13OH + 12.75H + 2+ – 2– 340 ⇌ 0.95Na + 5.90Mg + 0.99Al(OH)4 + 7.07H2SiO4 + 5.86H2O (l0) 341 342 The transformation to the above semi-isocoulombic reaction is achieved by
343 combination of Reaction (7) with the following reaction,
344
+ – 345 12.75 × [H2O(l) = H + OH ] (11)
346
347 In the one-term isocoulombic approach for extrapolation, the equilibrium
348 constants for Reaction (10) at the temperatures of interest are first calculated. Then, the
349 solubility constants for saponite for Reaction (7) at the respective temperatures are
350 retrieved by adding the equilibrium constants for Reaction (11) to those for Reaction
351 (10). The equilibrium constants for Reaction (11) at the temperatures of interest are
352 taken from EQ3/6 database, DATA0.YPF. The extrapolated solubility constants for
353 saponite at temperatures close to 80 oC obtained in this way are listed in Table 4.
17 354 The saturation index, for the experimental solution of Debure et al. (2016) (see
355 their Table 3) at 90 oC is calculated (Table 5) with the solubility constant of saponite at
356 90 oC. Note that the chemical composition of my synthesized saponite is similar to that
357 reported by Debure et al. (2016) (“Phyllosilicate-Mg A”; see their Table 7). The
358 calculated saturation index is 0.54, indicating that the solution is saturated or slightly
359 supersaturated with saponite (Table 5). This is in good agreement with the experimental
360 observations that Mg-phyllosilicate did form in their experiment. As mentioned above,
361 the calculated saturation index is 0.54 (Table 5). Statistically speaking, the predictions
362 could encompass the exact saturation when the uncertainty associated with the solubility
363 constant of saponite at 90 oC (–67.6 ± 2.5, Table 4) is taken into consideration.
364 Thien et al. (2012) conducted glass corrosion experiments in synthetic ground
365 water (SGW) at 50 oC relevant to geological disposal of HLW. The SGW has the
366 chemical compositions that represent the ground water in equilibrium with the Callovo-
367 oxfordian argillite formation at Bure underground laboratory. The glasses they used were
368 AVM 6 and AVM 10 glasses; both of them containing magnesium as one of the
369 components. The AVM 6 glass was more enriched in SiO2 than the AVM 10 glass. The
370 Mg-phyllosilicate (saponite) was precipitated in their experiments involving the AVM 6
371 glass.
372 Using the extrapolated solubility constant for saponite at 50 oC (Table 4), the
373 author calculated the saturation index for the experimental compositions of Thien et al.
374 (2012) involving the AVM 6 glass with the SGW at 50 oC. The calculated saturation
375 index is 0.71 (Table 6), indicating that saponite has a thermodynamic driving force for its
18 376 formation from the experimental solution. This is consistent with the experimental
377 observations.
378 IMPLICATIONS AND CONCLUDING REMARKS
379 One important implication from this study is that when a nuclear waste glass is
380 corroded in Mg-bearing solutions, Mg-saponite forms as a corrosion product, and the
381 resulting solutions are close to be in equilibrium with Mg-saponite. Therefore, the
382 solubility constants of Mg-saponite should be in the database as an essential part for
383 prediction of the corrosion rates as a function of the solution chemistry in equilibrium
384 with Mg-saponite.
385 Wilson et al. (2006) used the method proposed by Vieillard (2000) to estimate the
386 Gibbs free energies of formation for smectites including Na-saponite with the
387 stoichiometry of Na0.70(Mg6.00)[(Si7.30Al0.70)O20](OH)4. Notice that Wilson et al. (2000)
388 used O10(OH)2 per formula unit for Na-saponite. To be consistent with O20(OH)4 per
389 formula unit used in this study, their values are converted to those referring to O10(OH)2
390 per formula unit. They calculated the equilibrium constants for the dissolution of Na-
391 saponite according to the following reaction,
392
+ + 2+ 393 Na0.70(Mg6.00)[(Si7.30Al0.70)O20](OH)4 + 14.8H ⇌ 0.70Na + 6 Mg
3+ 394 + 0.70Al + 7.30SiO2(aq) + 9.4H2O(l) (12)
395
0 o 396 The log10 K for Reaction (12) at 80 C estimated by Wilson et al. (2006) is 50.95
397 (uncertainty not provided, and the same is true with the following citations of the values
398 from Wilson et al., 2006). The stoichiometry of their Na-saponite is very close to that of
19 399 the saponite studied in this work, and therefore their estimated value can be compared
400 with the value determined in this study. In order to do a direct comparison with the
401 equilibrium constant at 80 oC determined in this study, Reaction (7) is transformed into
402 the following form,
403
+ + 2+ 404 Na0.95Mg5.90[(Si7.07Al0.99)O20](OH)4 + 15.72H ⇌ 0.95Na + 5.90Mg 405
3+ 406 + 0.99Al + 7.07SiO2(aq) + 9.86H2O(l) (13) 407
408 by combining Reaction (7) with the following reactions,
409
+ – 410 H2O(l) = H + OH (14)
411
+ 2– 412 SiO2(aq) + 2H2O(l) = 2H + H2SiO4 (15)
413
3+ – + 414 Al + 4H2O(l) = Al(OH)4 + 4H (16)
415
416 The equilibrium constants for Reactions (14) through (16) are from the EQ3/6 Data0.YPF
0 417 database, Xiong (2013), and Xiong (2014), respectively. Accordingly, the log10 K for
o 0 418 Reaction (13) at 80 C is 70.30 ± 2.08. The log10 K estimated by Wilson et al. (2006)
419 differs from the experimentally determined value by ~20 orders of magnitude.
0 o 420 The estimated log10 K for Reaction (12) at 90 C is 48.75 (Wilson et al. 2006), in
0 421 comparison with the extrapolated log10 K of 70.8 ± 2.5, based on the experimentally
20 0 422 determined value. When the estimated log10 K is used to calculate the saturation index
Q 423 (log ) of saponite for the solution chemistry from Debure et al. (2016) at 90 oC, the K
Q 424 saturation index is too high ( log > 12) to be consistent with the experimental K
0 o 425 observations. Similarly, the estimated log10 K for Reaction (12) at 50 C is 58.35
0 426 (Wilson et al. 2006), whereas the extrapolated log10 K is 70.4 ± 2.5. When the
0 o 427 estimated log10 K is applied to the solution chemistry at 50 C from Thien et al. (2012),
Q 428 the saturation index is too high ( log > 12) to be consistent with the experimental K
429 observations. Consequently, in light of the experimental results presented in this work,
430 the estimation method needs to be revised to reconcile with the experimentally
431 determined values of this work and the experimental observations of Thien et al. (2012)
432 and Debure et al. (2016).
433 In addition, there is a solid phase of saponite-Na in the ThermoChimie database
434 (Giffaut et al., 2014; Blanc et al., 2015). The saponite-Na in the database has the
435 following stoichiometry on the basis of O20(OH)4 per formula unit:
0 436 Na0.68(Mg6.00)[(Si7.32Al0.68)O20](OH)4. The solubility constant (log10 K ) for dissolution
437 of the saponite-Na at 25 oC according to the following reaction
+ + 2+ 438 Na0.68(Mg6.00)[(Si7.32Al0.68)O20](OH)4 + 14.72H ⇌ 0.68Na + 6 Mg
3+ 439 + 0.68Al + 7.32H4SiO4(aq) + 9.4H2O(l) (12)
440 is 57.28 ± 7.30 based on the data set of 10a Version of September 26, 2018 (ANDRA,
441 2021). Notice that the error estimates are calculated based on the error percentage
21 442 relative to the basis of O10(OH)2 per formula unit in the original data set. The
443 extrapolated value at 25 oC for Reaction (13) is 72 ± 5, based on the value listed in
444 Table 4 in this study. When the quoted uncertainties and differences in stoichiometry are
445 taken into consideration, the estimated value from the ThermoChimie can be considered
446 to agree with the experimentally-based value at 25 oC. This is encouraging for the
447 estimation method, and it can be recalibrated with the experimental data at 25 oC
448 obtained in this study. In this way, the estimation method will be better to reconcile with
449 the experimental data and experimental observations, especially at elevated temperatures,
450 noted for the comparisons with the estimated values from Wilson et al. (2006), as the
451 ThermoChimie database and Wilson et al. (2006) use the same or very similar estimation
452 method.
453 In summary, the solubility constant of saponite with a stoichiometry of
o 454 Na0.95(Mg5.90Al0.06)[(Si7.07Al0.93)O20](OH)4 at 80 C has been determined in this work.
455 Then, the author extrapolated this value to other temperatures close to 80 oC (i.e., 50 oC,
456 60 oC, 70 oC, 90 oC, and 100 oC) by employing the one-term isocoulombic approach. The
Q 457 author calculated the saturation indexes, log , for the solution chemistry from the glass K
458 corrosion experiments in the presence of a Mg-source at 50 oC and 90 oC from different
459 researchers. My calculated saturation indexes suggest that the solutions are
460 saturated/slightly-supersaturated with saponite, which are in close agreement with the
461 experimental observations at various temperatures. This suggests that the alteration
462 products containing Mg formed when glasses are corroded in a solution in the presence of
463 a Mg-source may control the chemical compositions, including hydrogen ion
464 concentrations, of the solutions in contact with the glasses. As the compositions of
22 465 natural saponite vary to some degree, the results are best applied to the situations where
466 the compositions of saponite are similar to those of synthetic saponite, such as the
467 saponite from Allt Ribhein, Skye.
468
469 Acknowledgements
470 Sandia National Laboratories is a multi-mission laboratory operated by National
471 Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of
472 Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear
473 Security Administration under contract DE-NA-0003525. This research is funded by Salt
474 R&D programs administered by the Office of Nuclear Energy (NE) of the U.S
475 Department of Energy. The views expressed in the article do not necessarily represent
476 the views of the U.S. Department of Energy or the United States Government. This paper
477 is published with the release number SAND2019-3740J. The author gratefully
478 acknowledges the laboratory assistance from Leslie Kirkes, Jandi Knox, Cassie Marrs,
479 Heather Burton, and Dick Grant. The author is grateful to the two journal reviewers for
480 their insightful and thorough reviews. Their reviews helped to improve the manuscript
481 significantly. The author thanks the Associate Editor (AE), Dr. Daniel Neuville, for his
482 editorial efforts, and the Editor, Dr. Hongwu Xu, for his editorial comments and his time.
483
484
23 485 REFERENCES 486 487 Abdelouas, A., Crovisier, J.-L., Lutze, W., Grambow, B., Dran, J.-C. & Müller, R. (1997) 488 Surface layers on a borosilicate nuclear waste glass corroded in MgCl2 solution. 489 Journal of Nuclear Materials 240, 100–111. 490 ANDRA, (2021) www.thermochimie-tbd.com/pages/species-data.php. Accessed on 491 April 19, 2021. 492 April, R.H. & Keller, D.M. (1992) Saponite and vermiculite in amygdales of the Granby 493 basaltic tuff, Connecticut Valley. Clay and Clay Minerals 40, 22–31. 494 Aréna, H., Godon, N., Rébiscoul, D., Podor, R., Garcès, E., Cabie, M. & Metre, J.-P. 495 (2016) Impact of Zn, Mg, Ni and Co elements on glass alteration: Additive effects. 496 Journal of Nuclear Materials 470, 55–67. 497 Aréna, H., Godon, N., Rébiscoul, D., Frugier, P., Podor, R., Garces, E., Cabie, M. & 498 Mestre, J.-P. (2017) Impact of iron and magnesium on glass alteration: 499 Characterization of the secondary phases and determinations of the solubility 500 constants. Applied Geochemistry 82, 119–133. 501 Bickmore, B.R., Nagy, K.L., Young, J.S., & Drexler, J.W. (2001) Nitrate-cancrinite 502 precipitation on quartz sand in simulated Hanford tank solutions. Environmental 503 Science and Technology 35, 4481–4486. 504 Bishop, J.L., Loizeau, D., McKeown, N.K., Saper, L., Dyar, M.D., Des Marais, D.J., 505 Parente, M. & Murchie, S.L. (2013) What the phyllosilicates at Mawrth Vallis catell 506 us about possible habitability on early Mars? Planetary and Space Science 86, 130– 507 149. 508 Blanc, P., Vieillard, P., Gailhanou, H., Gaboreau, S., Marty, N., Claret, F., Made, B. & 509 Giffaut, E., 2015. ThermoChimie database developments in the framework of 510 cement/clay interactions. Applied Geochemistry 55, 95-107. 511 Bristow, T.F., Bish, D.L., Vaniman, D.T., Morris, R.V., Blake, D.F., Grotzing, J.P., 512 Rampe, E.B., Crisp, J.A., Achilles, C.N., Ming, D.W. & Ehlmann, B.L. (2015) The 513 origin and implications of clay minerals from Yellowknife Bay, Gale Crater, Mars. 514 American Mineralogist 100, 824–836. 515 Cahoon, H. P. (1954) Saponite near Milford, Utah. American Mineralogist 19, 222–230. 516 Calvin, W.M., Lautze, N., Moore, J., Thomas, D., Haskins, E. & Rasmussen, B.P. (2020) 517 Petrographic and spectral study of hydrothermal mineralization in drill core from 518 Hawaii: A potential analog to alteration in the martian subsurface. American 519 Mineralogist 105, 1297–1305. 520 Cuevas, J., De La Villa, V., Ramirez, S., Petit, S., Meunier, A. & Leguey, S. (2003) 521 Chemistry of Mg smectites in lacustrine sediments from the Vicalvaro sepiolite, 522 Madrid Neogene Basin (Spain). Clay and Clay Minerals 51, 457–472. 523 Curti, E., Crovisier, J. L., Morvan, G. & Karpoff, A. M. (2006) Long-term corrosion of 524 two nuclear waste reference glasses (MW and SON68): A kinetic and mineral 525 alteration study. Applied Geochemistry 21, 1152–1168.
24 526 Debure, M., De Windt, L., Frugier, P., Gin, S. & Vieillard, P. (2016) Mineralogy and 527 thermodynamic properties of magnesium phyllosilicates formed during the alteration 528 of a simplified nuclear glass. Journal of Nuclear Materials 475, 255–265. 529 Desprairies, A., Tremblay, P. & Laloy, C. (1989) Secondary mineral assemblage in a 530 volcanic sequence drilled during ODP Leg 104 in the Norwegian Sea. Proceedings of 531 the Ocean Drilling Program, Scientific Results, Vol. 104, 397–409. 532 Eberl, D.D., Whitney, G. & Khoury, H. (1978) Hydrothermal reactivity of smectite. 533 American Mineralogist 63, 401–409. 534 Fleury, B., Godon, N., Ayral, A. & Gin, S. (2013) SON68 glass dissolution driven by 535 magnesium silicate precipitation. Journal of Nuclear Materials 442, 17–28. 536 Frugier, P., Martin, C., Ribet, I., Advocat, T. & Gin, S. (2005) The effect of composition 537 on the leaching of three nuclear waste glasses: R7T7, AVM, and VRZ. Journal of 538 Nuclear Materials 346, 194–207. 539 Garvie, L.A.J. & Metcalfe, R. (1997) A vein occurrence of co-existing talc, saponite, and 540 corrensite, Builth Wells, Wales. Clay Minerals 32, 223–240. 541 Gaucher, E. C., Tournassat, C., Pearson, F. J., Blanc, P., Crouzet, C., Lerouge, C. & 542 Altmann, S. (2009) A robust model for pore-water chemistry of clayrock. Geochimica 543 Cosmochimica Acta 73, 6470–6487. 544 Giffaut, E., Grivé, M., Blanc, P., Vieillard, P., Colàs, E., Gailhanou, H., Gaboreau, S., 545 Marty, N., Made, B. & Duro, L., 2014. Andra thermodynamic database for 546 performance assessment: ThermoChimie. Applied Geochemistry, 49, pp.225-236. 547 Grambow, B. & Müller, R. (1989) Chemistry of glass corrosion in high saline brines. 548 Materials Research Society Symposium Proceedings 176, 229–240. 549 Grambow, B. & Strachan, D. M. (1984) Leach testing of waste glasses under near- 550 saturation conditions. Materials Research Society Symposium Proceedings, 623–634. 551 Gu, Y., Gammons, C.H. & Bloom, M.S. (1994) A one-term extrapolation method for 552 estimating equilibrium constants of aqueous reactions at elevated temperatures. 553 Geochimica et Cosmochimica Acta 58, 3545–3560. 554 Güven, N. (1990) Longevity of bentonite as buffer material in a nuclear-waste repository. 555 Engineering Geology 28, 233–247. 556 Harrison, M.T. (2014) The effect of composition on short- and long-term durability of 557 UK HLW glass. Procedia of Materials Science 7, 186–192. 558 He, H.-P., Li, T., Tao, Q., Chen, T.-H., Zhang, D., Zhu, J.-X., Yuan, P. & Zhu, R.-L. 559 (2014) Aluminum ion occupancy in the structure of synthetic saponites: Effect on 560 crystallinity. American Mineralogist 99, 109–116. 561 Hicks, L.J., Bridges, J.C. & Gurman, S.J. (2014) Ferric saponite and serpentine in the 562 nakhlite martian meteorites. Geochimica et Cosmochimica Acta 74, 4605–4611 563 Hund, F. (1984) Nitrate-cancrinite, thiosulfate-cancrinite, sulfate-cancrinite, and sulfide- 564 cancrinite. Z. Anorg. Allg. Chem, 509, 153–160.
25 565 Kadir, S. & Akbulut, A. (2003) The geology and origin of sepiolite, palygorskite and 566 saponite in Neogene lacustrine sediments of the Serinhisar-Acipayam Basin, Denizli, 567 southwestern Turkey. Clay and Clay Minerals 51, 279–292. 568 Kirkes, L. & Xiong, Y.-L. (2018) Experimental Determination of Brucite Solubility in 569 NaCl Solutions at Elevated Temperatures. In Xiong, Y.-L., ed., Solution Chemistry: 570 Advances in Research and Applications, pp. 48-68, Nova Science Publishers, New 571 York 572 Kohyama, N., Shimoda, S. & Sudo, T. (1973) Iron-rich saponite (ferrous and ferric 573 forms). Clay and Clay Minerals 21, 229–237. 574 Kong, X.Z., Tutolo, B.M. & Saar, M.O. (2013) DBCreate: A SUPCRT92-based program 575 for producing EQ3/6, TOUGHREACT, and GWB thermodynamic databases at user- 576 defined T and P. Computers & Geosciences 51, 415–417. 577 Lichtner, P.C. & Felmy, A.R. (2003) Estimation of Hanford SX tank waste compositions 578 from historically derived inventories. Computers & Geosciences 29, 371–383. 579 Liu, Q.Y., Xu, H.-W. & Navrosky, A. (2005) Nitrate cancrinite: Synthesis, 580 characterization, and determination of the enthalpy of formation. Microporous and 581 Mesoporous Materials 87, 146–152. 582 Luckscheiter, B. & Nesovic, M. (1998) Long term corrosion behaviour of the WAK- 583 HLW glass in salt solutions. Waste Management 17, 429–436. 584 Mackenzie, R.C. (1957) Saponite from Allt Ribhein, Fiskavaig Bay, Skye. Mineralogical 585 Magazine and Journal of the Mineralogical Society 31, 672–680. 586 Maeda, T., Ohmori, H., Mitsui, S. & Banba, T. (2011) Corrosion behavior of simulated 587 HLW glass in the presence of magnesium ion. International Journal of Corrosion 588 2011, Article ID 796457, 6 pages. 589 Mesmer, R.E. & Holmes, H.F. (1992) pH, definition and measurement at high 590 temperatures. Journal of Solution Chemistry 21, 725–744. 591 Nishri, A., Herbert, H. J., Jockwer, N. & Stichler, W. (1988) The geochemistry of brines 592 and minerals from the Asse Salt Mine, Germany. Applied Geochemistry 3, 317–332. 593 Post, J.L. (1984) Saponite from near Ballarat, California. Clay and Clay Minerals 32, 594 147–153. 595 Schuessler, W., Kienzler, B., Wilhelm, S., Neck, V. & Kim, J.I. (2001) Modeling of Near 596 Field Actinide Concentrations in Radioactive Waste Repositories in Salt Formations: 597 Effect of Buffer Materials. Materials Research Society Symposium Proceedings 663, 598 791–798. 599 Shao, H. & Pinnavaia, T.J. ( 2010) Synthesis and properties of nanoparticle forms 600 saponite clay, cancrinite zeolite and phase mixtures thereof. Microporous and 601 Mesoporous Materials 133, 10–17. 602 Sőhnel, O. & Novotný, P. (1985) Densities of aqueous solutions of inorganic substances. 603 Elsevier, New York, 335 p.
26 604 Strachan, D. M. (1983) Results from long-term use of the MCC-1 static leach test 605 method. Nuclear and Chemical Waste Management 4, 177–188. 606 Strachan, D. M., Krupka, K. M. & Grambow, B. (1984) Solubility interpretations of leach 607 tests on nuclear waste glass. Nuclear and Chemical Waste Management 5, 87–99. 608 Sueoka, Y., Yamashita, S., Kouduka, M. & Suzuki, Y. (2019). Deep microbial 609 colonization in saponite-bearing fractures in aged basaltic crust: Implications for 610 subsurface life on Mars. Frontiers in microbiology 10, Article 2793, 8 pages. 611 Tao, Q., Zeng, Q., Chen, M., He, H. & Komarneni, S., 2019. Formation of saponite by 612 hydrothermal alteration of metal oxides: Implication for the rarity of hydrotalcite. 613 American Mineralogist: Journal of Earth and Planetary Materials, 104(8), pp.1156- 614 1164. 615 Thien, B. M. J., Godon, N., Hulbert, F., Angeli, F., Gin, S. & Ayrol, A. (2010) Structural 616 identification of a tri-octahedral smectite formed by the aqueous alteration of a 617 nuclear glass. Applied Clay Science 49, 135–141. 618 Thien, B. M. J., Godon, N., Ballestero, A., Gin, S. & Ayrol, A. (2012) The dual effect of 619 Mg on the long-term alteration rate of AVM nuclear waste glass. Journal of Nuclear 620 Materials 427, 297–310. 621 U.S. National Academy of Sciences Committee on Waste Disposal. (1957) The Disposal 622 of Radioactive Waste on Land. Publication 519. Washington, DC: National Academy 623 of Sciences–National Research Council. 624 Utton, C.A., Hand, R.J., Hyatt, N.C., Swanton, S.W. and Williams, S.J. (2013) 625 Formation of alteration products during dissolution of vitrified ILW in a high-pH 626 calcium-rich solution. Journal of Nuclear Materials 442, 33–45. 627 Vieilard, P., 2000. A new method for the prediction of Gibbs free energies of formation 628 of hydrated clay minerals based on the electronegativity scale. Clays Clay Minerals 629 48, 459–473. 630 Wilson, J., Savage, D., Cuadros, J., Shibata, M. & Ragnarsdottir, K.V. (2006) The effect 631 of iron on montmorillonite stability. (I) Background and thermodynamic 632 considerations. Geochimica et Cosmochimica Acta 70, 306–322. 633 Wolery, T.J. & Jarek, R.L. (2003) Software user’s manual EQ3/6 (version 8.0). Sandia 634 National Laboratories, Albuquerque/New Mexico. 635 Wolery, T.J., Xiong, Y.-L. & Long, J. (2010) Verification and Validation Plan/Validation 636 Document for EQ3/6 Version 8.0a for Actinide Chemistry, Document Version 8.10. 637 Carlsbad, NM: Sandia National laboratories. ERMS 550239. 638 Wood, S.A., Palmer, D.A., Wesolowski, D.J. & Bénézeth, P. (2002) The aqueous 639 geochemistry of the rare earth elements and yttrium. Part XI. The solubility of 3+ o 640 Nd(OH)3 and hydrolysis of Nd from 30 to 290 C at saturated water vapor pressure 641 with in-situ pHm measurement. In Hellmann, R. and Wood, S.A., ed., Water-Rock 642 Interactions, Ore Deposits, and Environmental Geochemistry: A Tribute to David 643 Crerar, Special Publication 7, The Geochemical Society, pp. 229–256.
27 644 Xiong, Y.-L. (2008) Thermodynamic properties of brucite determined by solubility 645 studies and their significance to nuclear waste isolation. Aquatic Geochemistry 14, 646 223–238. 647 Xiong, Y.-L. (2011) WIPP Verification and Validation Plan/Validation Document for 648 EQ3/6 Version 8.0a for Actinide Chemistry, Revision 1, Document Version 8.20. 649 Supersedes ERMS 550239. Carlsbad, NM. Sandia National Laboratories. ERMS 650 555358. 651 Xiong, Y.-L. (2013) A thermodynamic model for silica and aluminum in alkaline 652 solutions with high ionic strength at elevated temperatures up to 100 oC: Applications 653 to zeolites. American Mineralogist 98, 141–153.
654 Xiong, Y.-L. (2014) A Pitzer model for the Na-Al(OH)4-Cl-OH system and solubility of 655 boehmite (AlOOH) to high ionic strength and to 250oC. Chemical Geology 373, 37– 656 49. 657 Xiong, Y.-L. (2015) Experimental determination of lead carbonate solubility at high ionic 658 strengths: a Pitzer model description. Monatshefte für Chemie-Chemical Monthly 659 146, 1433–1443. 660 Xiong, Y.-L. (2016) Solubility constants of hydroxyl sodalite at elevated temperatures 661 evaluated from hydrothermal experiments: Applications to nuclear waste isolation. 662 Applied Geochemistry 42, 1393–1403. 663 Xiong, Y.-L. & Lord, A.S. (2008) Experimental investigations of the reaction path in the 664 MgO–H2O–CO2 system in solutions with various ionic strengths, and their 665 applications to nuclear waste isolation. Applied Geochemistry 23, 1634–1659. 666 Xiong, Y.-L., Deng, H.-R., Nemer, M. & Johnsen, S. (2010) Experimental determination 667 of the solubility constant for magnesium chloride hydroxide hydrate 668 (Mg3Cl(OH)5×4H2O, phase 5) at room temperature, and its importance to nuclear 669 waste isolation in geological repositories in salt formations. Geochimica et 670 Cosmochimica Acta 74, 4605–4611. 671 Xiong, Y.-L. (2020) Experimental determination of the solubility constant of 672 kurnakovite. MgB3O3(OH)5•5H2O. American Mineralogist 105, 977–983. 673 Xu, T., Pruess, K. & Brimhall, G. (1999) An improved equilibrium-kinetics speciation 674 algorithm for redox reactions in variably saturated subsurface flow systems. 675 Computers & Geosciences 25, 655–666. 676 Yang, T., Pusch, R., Knutsson, S. & Liu, X.-D. (2014) The assessment of clay buffers for 677 isolating highly radioactive waste. WIT Transactions on Ecology and The 678 Environment 180, 403–413. 679 Zhang, Z., Gan, X., Wang, L. & Xing, H. (2012) Alteration Development of the 680 Simulated HLW Glass at High Temperature in Beishan Underground Water. 681 International Journal of Corrosion 2012, Article ID 924963, 7 pages. 682
28 683 Figure Captions 684 685 686 Figure 1. XRD pattern of the solubility-controlling phases in the experiment performed 687 in this work, compared with those of saponite 15A and NO3-cancrinite. 688 689 690 Figure 2. SEM images and EDS analyses for the solid phases from the experiment 691 performed in this study. A–C are SEM images with the respective EDS analyses: A. 692 magnification at 6,500 times; B. magnification at 5,500 times; and C. magnification at 693 3,700 times. 694 695 – 696 Figure 3. A plot showing total molal concentrations of Al(III), Mg(II), Na(I), NO3 , and 697 Si(IV) as a function of experimental time in an experiment approaching equilibrium from 698 the direction of supersaturation at 80 oC. 699 700 701 702 703 704
29 705 706 Table 1. Chemical compositions of saponite synthesized in this work in comparison with 707 those of natural saponite. 708 709 Synthetic Natural Natural Saponite, Saponite, Saponite, This Work, Ballarat, Milford, wt% California, Utah, Natural Saponite, Allt Ribhein, Oxide wt% A wt% B Fiskavaig Bay, Skye, wt% C SiO2 46.80 51.26 50.01 43.62 TiO2 0.00 0.09 0.00 0.00 Al2O3 5.56 4.42 3.89 5.50 Fe2O3 0.00 1.14 0.21 0.66 FeO 0.00 --- 0.00 --- MnO 0.00 0.03 0.00 0.06 MgO 26.22 23.54 25.61 24.32 CaO 0.00 1.25 1.31 2.85 Na2O 3.27 1.14 0.00 0.08 K2O 0.00 0.18 0.00 0.04 D H2O (LOI) 19.26 16.66 17.30 22.90 710 A III 711 Post (1984), (Ca,Na,K)0.75(Mg5.17Al0.31Fe 0.13)[Si7.55Al0.45]O20(OH)4. The 712 stoichiometry is calculated in this work on the basis of O20(OH)4 per formula unit. 713 B III 714 Cahoon (1954), (Ca,Na,K)0.42(Mg5.72Al0.19Fe 0.02) [Si7.50Al0.50]O20(OH)4. The 715 stoichiometry is calculated in this work based on O20(OH)4 per formula unit.. 716 C III 717 Mackenzie (1957), (Na, K, Ca)1.02(Mg5.84, Mn0.01, Al0.03, Fe 0.08)Si6.99Al1.01O20(OH)4. 718 The stoichiometry was calculated by Mackenzie (1957). I obtained almost the same 719 formula based on the compositions provided. 720 721 D Based on TGA analysis up to 700 oC. 722 723 724
30 725 726 Table 2. Experimental data from the solubility experiment regarding the equilibrium 727 between nitrate cancrinite and saponite produced in this study at 80.0 ± 0.5 oC. 728
Experimental Experimental C A B B B B m - D Number time, days pHm mNa mSMg(II) mSAl(III) mSSi(IV) NO3 mOH- SAP-80-1 64 12.58 1.38 1.24E-05 2.94E-04 1.66E-02 6.72E-01 0.674 85 12.56 1.42 1.11E-05 2.78E-04 1.99E-02 7.01E-01 0.674 92 12.61 1.44 1.50E-05 1.94E-04 3.51E-02 6.91E-01 0.674 729 A Measured pH readings at 64, 85, 92 days were 12.43, 12.40, and 12.45, respectively, at 730 the experimental temperature. The pHm values are calculated by applying the 731 correction factor at 80 oC from Kirkes and Xiong (2018) at the ionic strength of the 732 experiment. 733 B Analyzed with ICP-AES 734 C Calculated based on charge balance 735 D Based on the initial NaOH concentration. 736 737 738
31 739 740 Table 3. Equilibrium constants of saponite at 80 ± 0.5 oC determined in this work 741 A 0 Reactions log10 Q log10 K – + Na0.95Mg5.90[(Si7.07Al0.99)O20](OH)4 + 2NO3 + 7.05Na + – 2+ 5.01Al(OH)4 ⇌ Na8(Al6Si6O24](NO3)2•4H2O + 5.90Mg + –23.52 ± –31.25 ± – 2– 9.62OH + 1.07H2SiO4 + 2.14H2O(l) (1) 1.96 (2s) 1.96 (2s) – Na8(Al6Si6O24](NO3)2•4H2O + 12OH + 8H2O(l) –37.99 ± + – 2– – ⇌ 8Na + 6Al(OH)4 + 6H2SiO4 + 2NO3 (4) 0.70 (2s) B – Na0.95Mg5.90[(Si7.07Al0.99)O20](OH)4 + 2.38OH + 5.86H2O(l) –69.24 ± + 2+ – 2– ⇌ 0.95Na + 5.90Mg + 0.99Al(OH)4 + 7.07H2SiO4 (7) 2.08 (2s) C 742 A At ionic strength of 1.4 mol•kg–1 743 744 B Based on a linear interpolation of the values at 89 oC (–36.20) from Bickmore et al. 745 (2001) and at 75 oC (–39.03) from Lichtner and Felmy (2003). 746 747 C A combination of the reaction in Row 2 with that in Row 3 leads to the reaction in Row 748 4 and its corresponding equilibrium constant. 749 750 751 Table 4. Extrapolated equilibrium constants of saponite with the stoichiometry 752 determined in this work at other temperatures close to 80oC 753 o 0 Reactions T C log10 K 25 –80 ± 5 C 50 –74.7 ± 2.5 C 60 –72.8 ± 2.5 C – Na0.95Mg5.90[(Si7.07Al0.99)O20](OH)4 + 2.38OH + 5.86H2O(l) + 2+ – 2– ⇌ 0.95Na + 5.90Mg + 0.99Al(OH)4 + 7.07H2SiO4 70 –70.96 ± 2.5 B 80 –69.24 ± 2.08 (2s) A 90 –67.6 ± 2.5 C 100 –66.1 ± 2.5 C 754 A Determined in this study. 755 B One-term isocoulombic extrapolation based on the experimental value at 80 oC. 756 C Linear extrapolation in the space of log K vs. 1/T where T is in absolute temperature in 757 K, based on the values at 70 oC and 80 oC. 758 759 760
32 761 Table 5. Chemical compositions of the solution in which the international simple glass 762 was corroded at 90 oC Concentration SSi SB SNa SAl SCa SMg mmolar A 6.00E- 5.00E- 5.00E- 1.49E+00 1.1561E+02 6.298E+01 02 02 02 mmolal B 6.24E- 5.20E- 5.20E- 1.55E+00 1.20E+02 6.55E+01 02 02 02 Saturation state with respect to saponite, Na0.95Mg5.90[(Si7.07Al0.99)O20](OH)4 Q log C K 0.5367 763 A From Debure et al. (2016); concentration unit, 10–3 mol•dm–3. 764 B This study, concentration unit, 10–3 mol•kg–1. Converted from the results from Debure 765 et al. (2016) by using a density of 0.15 mol•dm–3 NaCl at 90 oC to approximate the 766 density of the experimental solution in Debure et al. (2016). The density of 0.15 767 mol•dm–3 NaCl at 90 oC is from Sőhnel and Novotný (1985). Q 768 C In log , Q is ion activity product (IAP) with respect to saponite; K is the equilibrium K 769 constant defined by Reaction (7). In the calculation, pH was assumed to be constrained 770 by the charge balance. 771 772 773 774 775
33 776 777 Table 6. Chemical compositions of the solution in which the AVM 6 nuclear glass was 778 corroded in synthetic groundwater (SGW) at 50 oC Concentration SSi SB SNa SLi SAl mg/L A 19 786 2257 24 1.9 molar B 6.77E-04 7.27E-02 9.82E-02 3.46E-03 9.06E-05 molal C 6.86E-04 7.38E-02 9.96E-02 3.51E-03 9.19E-05 Concentration SMg SK SCa SCl SSO4 mg/L A 16 45 30 1491 1397 molar B 6.58E-04 1.15E-03 7.49E-04 4.21E-02 1.45E-02 molal C 6.68E-04 1.17E-03 7.59E-04 4.27E-02 1.48E-02 Saturation state with respect to saponite, Na0.95Mg5.90[(Si7.07Al0.99)O20](OH)4 Q log D K 0.7075 779 A From Thien et al. (2012), their experimental compositions involving the AVM 6 glass 780 with the SGW at 219 days and 50 oC; concentration unit, 10–3 g•dm–3. 781 B This study, concentration unit, mol• dm–3; converted from the results of 782 Thien et al. (2012). The calculated total ionic strength for the solution is 0.14 mol• 783 dm–3. 784 C This study, concentration unit, mol•kg–1. Converted from the results on molar units by 785 using a density of 0.14 mol•dm–3 NaCl at 50 oC to approximate the density of the 786 SGW in which AVM 6 nuclear glass was corroded at 50 oC. The density of 0.14 787 mol•dm–3 NaCl at 50 oC is from Sőhnel and Novotný (1985). 788 Q 789 D In log , Q is ion activity product (IAP) with respect to saponite; K is the equilibrium K 790 constant defined by Reaction (7). 791 792 793 Appendix A. The elements, their lowest limits of detection, or LLD (wt.%), the crystal 794 diffractometer used and counting times for EPMA analyses 795 Element 1-s LLD (wt.%) Crystal Counting time (s) Si 0.040 TAP A 20 sec Peak, 5 sec Background Al 0.027 TAP 20 sec Peak, 5 sec Background Mg 0.055 TAP 20 sec Peak, 5 sec Background Na 0.050 TAP 20 sec Peak, 5 sec Background 796 A TAP, Thallium Acid Phthalate. 797
34 798 Appendix B. Comparison of major XRD peaks of saponite synthesized in this study with
799 those of the saponite 15 Å standard (PDF-00-013-0086)*
Mg-saponite Standard, PDF-00-020-0964 Saponite, Synthesized in this study hkl d-spacing, Å Intensity hkl d-spacing, Å Intensity** (100) 4.57 50 (100) 4.572 166 (004) 3.67 60 (004) 3.666 214 (111) 2.58 20 (111) 2.577 242 (006) 2.42 20 (006) 2.421 217 (007) 2.09 30 (007) 2.089 82 (300) 1.53 90 (300) 1.534 111 (221) 1.32 40 (221) 1.322 76 800 *The major peaks with intensity ≥20 in the standard are compared with those in 801 the synthetic saponite. The significant numbers presented for d-spacing of the 802 synthesized saponite are one more than those presented in the PDF database for 803 comparison. It is obvious that the synthesized saponite has the identical d- 804 spacings when its significant numbers are rounded as the same significant 805 numbers as the PDF database does. 806 ** Experimental relative intensity 807 808 809 810 811 812
35 1500 Saponite and nitrate cancrinite synthesized in this work
1250
1000
750
500
Relative Relative Intensity Standard PDF 00-013-0086, Saponite-15A, Mg3(Si, Al)4O10(OH)2•4H2O
250
Standard PDF 00-038-0531, NO3-Cancrinite, Na8[Al6Si6O24](NO3)2•4H2O
0 10 20 30 40 50 60 70 80 90 2 theta 813 814 Figure 1. 815 816 817 818 819 820 821
36 822
823 824
825 826 827 828 Figure 2A. 829
37 830 831
832 833 834 835 Figure 2B. 836 837
38 838 839
840 841 842 Figure 2C. 843 844 845 846 847
39 858 1.0E+01 m = 0.674 mol•kg-1 Na OH-
1.0E+00
1 – - NO3 1.0E-01
1.0E-02 Si
1.0E-03
1.0E-04 Al Concentration, molal, mol•kgmolal, Concentration,
1.0E-05 Mg
1.0E-06 50 60 70 80 90 100 859 Experimental Time, Day 860 861 Figure 3. 862 863 864
42